Smoke detectors or particulate detectors known in the art project a beam of electromagnetic radiation into a medium or fluid that contains particulates. As the radiation, for instance visible light, infrared or ultraviolet radiation, is scattered by the particulates suspended in the transparent fluid or medium, a measurement of the light scattered away from the colimated beam of light is made using a light detection device. The measurement of the quantity of light scattered by the particulates in the medium provides an indication of the density or quantity of particulates suspended in the fluid.
A particulate or smoke detector known in tile art is shown in Meili, U.S. Pat. No, 4,166,960. Meili discloses a smoke detector including a radiation source that produces a directed radiation beam and a scattered radiation receiver arrangement having directional characteristics. An evaluation circuit is operatively connected with the receiver arrangement for delivering a signal whenever the quantity of scattered light exceeds a predetermined level. The radiation receivers observe two separate fields of view and output signals produced by the receivers are compared with one another.
In view of ever increasing concerns for the health of the environment, it is more desirable of late to measure the quantity of particulates emanating from an internal combustion engine. One such device for detecting particulates in an exhaust stack is disclosed in Dunkel, U.S. Pat. No. 4,647,780. The Dunkel device projects a light beam through an exhaust stack and a photodetector located on the opposite side of the stack from the light beam producing device produces an electronic signal corresponding to the quantity of light that passes through the smoke.
Other devices known to applicant relating to particulate or smoke detection are shown in the following U.S. patents: Schneider et al., U.S. Pat. No. 4,818,705; Boeke, U.S. Pat. No. 3,954,342; Hartmann et al., U.S. Pat. No. 4,126,396; Villaume et al., U.S. Pat. No. 4,040,743; Brekelmans, U.S. Pat. No. 4,559,813; Hasinger, U.S. Pat. No. 4,867,033; Hall, U.S. Pat. No. 4,583,859; Onoda et al., U.S. Pat. No. 4,260,883; Lord, U.S. Pat. No. 4,746,218; Kontani et al., U.S. Pat. No. 4,719,360; and Wyatt et al., U.S. Pat. No. 4,548,500.
None of the devices known to applicant includes a design configuration that properly compensates for variations in the intensity of the light beam projected into the fluid over an extended period of time. In other words, as the light producing device ages or optical components of the system accumulate dirt and grime, the loss of intensity of the light beam results in a steady degradation in the performance of the smoke detection devices shown in the prior art. Thus, a particulate or smoke detection device that compensates for deterioration in performance of the light producing device or transmissivity of the optical components is needed.